The Followin the full details about that error, I can send mail from UNIX (HP UNIX) box by using mailx command!!

ORA-29278: SMTP transient error: 421 Service not available
ORA-06512: at "SYS.UTL_SMTP", line 17
ORA-06512: at "SYS.UTL_SMTP", line 96
ORA-06512: at "SYS.UTL_SMTP", line 138
ORA-06512: at "INFOPUMP.SEND_MAIL", line 8
ORA-06512: at line 2